As miniaturization continues, micropumps are becoming more and more important. In practical use, micropumps may frequently use a particle filter at the inlet of the micropump, since otherwise particles existing in the pump chamber may cause leakage rates at the microvalves of the micropump, or particles might impede movement of a pump membrane. This may eventually cause the pump to fail.
In contrast to particles, most micropumps nowadays are tolerant toward gas bubbles (bubble-tolerant), so that gas bubbles need not be separated. Various gases are possible, the simplest case involving air bubbles.
Filters—no matter whether they are hydrophobic (liquid-repellant) or hydrophilic (liquid-attracting)—may frequently use a high pressure to be able to guide a gas bubble through a wetted filter. This applies to hydrophilic filters in particular. Frequently, a high pressure may also used for guiding, e.g., a drop of liquid through a hydrophobic filter. However, the elevated pressure that may be used for this is disadvantageous in terms of the flow resistance. Therefore, it is particularly for micropumps that the flow resistance should be influenced by the filter to as small a degree as possible.
In microfluidics, liquids are pumped through micropumps, and to avoid any contamination of the micropumps with particles, hydrophilic filters are mostly used in front of the inlet of the micropump. However, if a gas bubble is sucked in by the micropump at the wetted hydrophilic filters, the filter will be blocked by the gas bubble. However, micropumps are frequently not able to generate the high suction powers that may be used to suck the air bubble through the filter. The suction power (or suction pressure) that may be used may amount to up to one bar, for example. As a consequence, the system may fail completely.
In microfluidics, in the very frequent case that only small system pressures occur (such as in the so-called lab-on-a-chip applications, in micropumps, etc.), it is a general and almost fundamental problem that, on the one hand, particle filters are used, but, on the other hand, gas bubbles may block these filters with a very high pressure. The very high pressure results, for example, from the small pore size of the filters used, and the smaller the particles that are to be filtered out, the higher said pressure becomes. Since micropumps are very small, it is important to filter out even very small particles. However, in order that the small particles can be filtered out, the pore size should also be very small, which in turn increases the pressure that may be used to press gas bubbles, for example, through the filter. However, the micropumps used can only generate a limited pressure, which is frequently not sufficient to press the gas bubbles through the very small pores of the filter.
Conventional filters have a hydrophobic area and a hydrophilic area and are already known from conventional technology. For example, US 2003/0042211 discloses a known filter wherein a serial arrangement of hydrophobic and hydrophilic material is implemented to separate bubbles as well as particles from a liquid. What is also known are filters wherein gas bubbles may be removed from the flow path and passed on to the ambient air by using hydrophobic material. However, a disadvantage of said systems is that during operation with a micropump at the suction side, air bubbles may be sucked into the system from the environment by the hydrophobic material. U.S. Pat. No. 5,997,263 describes a further conventional filter for removing, or avoiding, any bubbles that may have become stuck at specific locations within the filter. However, the filter arrangement described is a one-dimensional filter and accordingly has a substantially higher flow resistance than a two-dimensional planar filter. In addition, with this one-dimensional filter implemented as a barrier, different hydrophobic and hydrophilic areas are implemented along the filter line, the duct cover as a whole being either hydrophobic or hydrophilic, however. This will also lead to an increase in the flow resistance.
Further bubble-tolerant particle filters are described in U.S. Pat. No. 4,278,084 and in GB 1510072 and are used for artificial feeding. However, for both filters it is useful to orient them such that the hydrophobic section comes to lie vertically above the hydrophilic one, so that any rising air bubbles will move toward the hydrophobic section due to gravity. It is only there that the air bubbles can pass the hydrophobic filter. The “proper” orientation of the filter is thus indispensable for the filter to function. In particular following a rotation by 180° about a horizontal axis, the filter will not function or its performance will be clearly poorer, since this leads to a marked increase in the flow resistance.
U.S. Pat. No. 3,523,408 A discloses a gas burner wherein a distance of 0.25 mm to 5 mm is provided between the essentially parallel liquid-repellant and liquid-wetting filter materials.
U.S. Pat. No. 5,190,524 A discloses a device for combining a plurality of liquid infusions to form a mixture. A chamber exhibits a plurality of separately closable inlets and one outlet for the mixture. One hydrophilic membrane and one hydrophobic membrane are provided within the chamber.
From U.S. Pat. No. 5,989,318 A, a device for separating water from a two-phase flow is known, wherein in one cavity a hydrophobic filter is provided for removing gas from the two-phase flow. In addition, a hydrophilic filter is provided to prevent water from exiting from a water outlet opening.
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EP 0489403 A2 describes a filter device comprising a housing and a microporous medium in the form of a synthetic polymeric microporous structure.
DE 1949038 A describes a separator for gases and liquids which has a liquid-repellant section arranged in front of an outlet for a gas, and a wettable section provided in front of an outlet for a liquid.
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